1. Field of the Invention
The present invention relates to a belt speed reducing apparatus for an electric power steering apparatus and an electric power steering apparatus.
2. Description of the Related Art
There is known an electric power steering apparatus for a vehicle for reducing a speed of a rotational output of a motor via a speed reducing mechanism to assist steering operation as a steering assisting force.
For example, in an electric power steering apparatus 500 shown in FIG. 41, a steering shaft 501 and a rack shaft 502 are brought in mesh with each other by a publicly-known pinion rack mechanism 503. Further, a motor shaft 505 of a motor 504 is rotatably supported by bearings 506, 507.
The rack shaft 502 is formed with a male screw portion (spiral groove) 508. A nut 510 rotatably supported by a bearing 509 is arranged on an outer side thereof. A ball 511 is inserted to fit between the mail screw portion 508 of the rack shaft 502 and the nut 510 to thereby constitute a ball screw mechanism 512.
A shaft 515 of an input gear 514 is coupled with an extended portion 513 of the motor shaft 505 by a spline coupling SP. The input gear 514 is brought in mesh with a middle gear 516 and the middle gear 516 is brought in mesh with an output gear 517. The output gear 517 is formed in a cylindrical shape and penetrated with the rack shaft 502 at inside thereof. Further, outer sides of both end portions in an axial direction of the output gear 517 are rotatably supported by bearings 518, 519, and a spline groove 520 is formed at an inner face in the cylindrical shape of the output gear 517.
Meanwhile, a spline projected streak 522 is formed at an outer side of an extended portion 521 at one end of the nut 510 of the ball screw mechanism 512 to couple with the spline groove 520 formed at the inner face of the output gear 517 by spline coupling SP.
In the above-described constitution, based on a steering torque of the steering shaft 501 detected by a torque detecting apparatus, not illustrated, a driving rotational force of the motor 504 driven by a control apparatus, not illustrated, is transmitted to the nut 510 of the ball screw mechanism 512 via the input gear 514, the middle gear 516, and the output gear 517. Further, the rack shaft 502 is moved in an axial direction by rotating the nut 510 to change a direction of a wheel to thereby carry out steering.
In a constitution of using the above-described gear speed reducing mechanism as a speed reducing mechanism, when a backlash of a mesh portion of the gear is large, there is brought about a drawback that when steering operation is reverted or the like, teeth striking sound is emitted at tooth faces. Further, when the backlash is excessively small, there is brought about a drawback that meshed rotating operation of the gear is not carried out smoothly and a steering feeling is deteriorated or the like. Therefore, it becomes necessary to set a magnitude of the backlash in a pertinent range.
For example, in a well-known worm gear speed reducing mechanism constituted by a worm and a worm wheel, as described in JP-A-10-297505 (hereafter, described as “Patent Reference 1”), there is proposed a constitution in which a housing of a worm gear speed reducing mechanism is divided into a housing on a side of a worm and a housing on a side of a worm wheel and a magnitude of a backlash is set in a pertinent range by adjusting an interval between the two housings.
Other than the above-described, in order to set a magnitude of a backlash in a pertinent range, there is a case in which a working dimensional accuracy in fabricating a gear is promoted, or a method referred to as matching of selecting a gear on a counterpart to be meshed in accordance with a state of a finish dimension of a fabricated gear or the like, however, these methods result in increasing fabrication cost, which is not preferable.
According to the electric power steering apparatus 500 having a constitution explained in reference to FIG. 41, the motor shaft 505 is arranged in parallel with the rack shaft 502, and in order to avoid interference between the motor 504 and the rack shaft 502, a distance between axis centers of the input gear 514 and the output gear 517 is constituted to be large. Thus, the input gear 514 and the output gear 517 are constituted to be brought in mesh with each other via the middle gear 516.
When the middle gear 516 is interposed in the gear speed reducing mechanism, a backlash between the input gear 514 and the middle gear 516 and a backlash between the input gear 516 and the output gear 517 need to adjust. However, when the gear is selected to determine by the above-described matching, the matching is carried out between the input gear 514 and the middle gear 516 and between the middle gear 516 and the output gear 517, which is much difficult to carry out.
Further, when a worm gear speed reducing mechanism is used, particularly in a constitution of using a saddle type worm as a worm, a three needles measuring method which is carried out in a normal worm cannot be used in measuring a finish dimension of the saddle type worm. Thus, it is very difficult to measure the dimension of the worm and therefore, matching is carried out in operation of integrating the speed reducing mechanism and there is brought about a drawback that a number of integrating steps is increased.
Further, according to a belt speed reducing apparatus constituted by an input pulley and an output pulley in place of the above-described input gear and output gear and a drive belt made to wrap on the pulleys, in order to maintain high transmitting efficiency and durability, it is requested to set a tension of the drive belt in a pertinent range, however, a dispersion among products of the drive belt is larger than that of the gear. Therefore, it is necessary to construct a constitution capable of easily adjusting a distance between axis centers of the input pulley and the output pulley.
Other than these, an electric power steering apparatus is constituted by a structure of integrating a motor to a housing of apparatus at a later stage and therefore, a motor shaft and a gear or a pulley are constituted by separate members. Thus, it is necessary to provide bearings to a motor shaft and a gear shaft or a pulley shaft and there is a drawback of increasing a number of parts and increasing fabrication cost.
Further, as a constitution of using a belt speed reducing apparatus, for example, according to JP-A-62-004673 (hereafter, referred to as “Patent Reference 2”), there is known a constitution of using a V belt of a friction type. Further, according to JP-UM-A-06-049489 (hereafter, described as “Patent Reference 3”), there is known a constitution of using a spur gear type belt (timing belt).
According to the belt speed reducing apparatus using the V belt described in Patent Reference 2, power is transmitted by friction and therefore, a large tension must be exerted to the belt in order to achieve the friction force. Therefore, a torque under no load is increased and therefore, there is a drawback that return of a steering wheel is increased. In contrast thereto, according to the belt speed reducing apparatus using the spur gear type belt described in Patent Reference 3, owing to a mesh transmitting type, a large tension is not needed and therefore, there is achieved an advantage of capable of reducing the no load torque. However, according to the spur gear type belt, particular operating sound is emitted in starting and in finishing engaging teeth on a side of the pulley and teeth on a side of the belt accompanied by rotation.
Although the problem can be resolved to some degree by reducing a size of the teeth of the belt, on the other hand, the teeth are weakened. Therefore, when a large force is exerted, there may be brought about a serious problem starting from shearing of the teeth, biting of the sheared teeth, locking of the speed reducing mechanism and finally, a progress to a state of being unable to carry out steering. Therefore, when the size of the teeth of the belt is reduced with an object of reducing the operating sound, it is necessary to widen a width of the belt, the speed reducing mechanism is large-sized unavoidably and therefore, layout of the speed reducing mechanism in a vehicle body becomes difficult.
Further, according to the electric power steering apparatus using the belt speed reducing apparatus, the tension exerted to the belt significantly controls a function of the electric power steering apparatus. In the case of the V belt of the friction type, when the tension is excessively larger than a proper value, an operating torque (friction under no load) is increased, return of the steering wheel is deteriorated and therefore, an adverse influence is effected on the steering wheeling. Conversely, when the tension is excessively smaller than the proper value, in the case of the V belt of the friction type, the friction force becomes deficient, slip is produced and therefore, power cannot be transmitted. Further, in the case of the spur gear type belt, a meshed state is deteriorated to pose a problem in durability. Therefore, when the belt is used in the speed reducing machine of the electric power steering apparatus, the tension of the belt needs to maintain at a value in the proper range.
The timing belt is widely adopted for driving a cam shaft of an engine or the like. The engine is rotated only in a constant direction and therefore, a side of exerting the tension and a side of loosening the tension on the belt are determined. Therefore, an automatic tensioner is provided on the side of loosening the tension, the tensioner is pressed to the belt by a pressing force of a predetermined elastic member or the like to constitute a proper value of the tension. An influence of error or variation of the distance between the axis centers and an elongation of the belt is substantially absorbed by elastically deforming the elastic member since the tensioner is displaced. Therefore, the belt tension adjusting mechanism using the elastic member achieves an advantage of capable of flexibly dealing with almost all of error and deformation of a constituting member.
However, a situation differs in a power transmitting system in which a rotational direction is changed both in regular and reverse direction. For example, when a timing belt is used in a speed reducing machine of an electric power steering apparatus, different from an engine, a rotational direction of the speed reducing machine is not constant. That is, the steering wheel is turned both to the right and to the left and therefore, also the rotational direction of the timing belt is changed in accordance therewith. A string-like member such as a belt can be loaded with only a tension force and therefore, one side of the belt is loaded with a tension for driving and other side thereof is not loaded therewith. When the rotational direction is changed, in belt driving, a side exerted with the tension and a side of loosening the tension are alternately changed in accordance with the change.
For example, in JP-A-2003-220958 (hereafter, described as “Patent Reference 4”), it is known to adjust a tension by using a tensioner. When an initial tension is exerted to a belt by using the tensioner by elastic force, a side provided with the tensioner is changed from a side of loosening the tension to a side exerted with the tension in accordance with a change in a rotational direction. Further, the tensioner is moved to a position balanced with the force in accordance with a change in the tension. During the time period, power cannot be transmitted, (or reduced) and therefore, a delay in transmitting power is brought about. The delay in transmitting power stays the same even when the tensioners are provided on both sides.
In a power steering apparatus, when such a delay in transmitting power is brought about, an assisting force is not transmitted during the time period even when an assisting motor is rotated. Since the steering wheel is not assisted, when the steering wheel is turned back, the steering wheel becomes heavy. At this occasion, a driver feels as if the steering wheel were caught by something and therefore, a steering feeling is deteriorated.
When a timing belt is used in a speed reducing machine of an electric power steering apparatus in order to avoid such a phenomenon, as described in JP-A-2003-220959 (hereafter, described as “Patent Reference 5”), it is necessary to exert an initial tension by utilizing an elastic force of the belt per se by adjusting a distance between axis centers of pulleys, or providing an idle pulley to adjust a center position of the idle pulley, that is, without using a tensioner by an elastic force.
At an initial stage when an automobile starts running, temperatures of respective parts and respective apparatus are low, however, when running is continued, the temperatures rise. The temperature rise is not exceptional also in a power steering apparatus. By the temperature rise, a total of the power steering apparatus is expanded and therefore, a change is produced in the distance between the axis centers of two pulleys on which the belt is made to wrap, or the position of the idle pulley. Although the belt per se is also expanded, generally, an amount thereof is small. As a general result of thermal expansion thereof, a change is produced in a tension of the belt. Particularly when the tensioner by an elastic force is not used, a difference in displacement of thermal expansion cannot be absorbed by displacement of the elastic member and therefore, almost all of the difference in displacement by thermal expansion significantly changes an average tension of the belt. This poses the above-described problem.
A general belt comprises rubber and a core wire and glass fiber is used as a material of the core wire. Further, aluminum is generally used in a housing of the electric power steering apparatus. Linear expansion coefficients of both members are respectively as follows.
Glass fiber: 0.5 through 0.7×10−5/° C.
Aluminum: 2.4×10−5/° C.
When a temperature range of using the electric power steering apparatus is set to −40° C. through 120° C., the above-described difference in the linear expansion coefficients cannot be disregarded. When the elastic member is not used, the tension of the belt is significantly varied in accordance with the change in the temperature owing to the difference to effect an adverse influence on a function of the electric power steering apparatus.